The present disclosure relates to an image forming apparatus using a photosensitive drum, and relates particularly to a method for removing moisture on a surface of the photosensitive drum.
In an image forming apparatus using an electrophotographic method, such as a copy machine, a printer, or a facsimile, a developing agent in powder form (hereinafter, referred to as toner) is mainly used, and, typically, a process is performed in which an electrostatic latent image formed on an image bearing member such as a photosensitive drum is visualized by using the toner in a developing device, and a toner image thus formed is transferred onto a recording medium and then subjected to fixing processing. A photosensitive drum is formed of a cylindrical base member and a photosensitive layer of ten to several tens of μm in thickness formed on a surface of the cylindrical base member. In terms of a main material constituting the photosensitive layer, photosensitive drums can be classified into an organic photosensitive member, a selenium arsenic photosensitive member, an amorphous silicon (hereinafter, abbreviated as a-Si) photosensitive member, and so on.
The organic photosensitive member, though being relatively low-cost, is susceptible to wear and thus requires frequent replacement thereof. Furthermore, the selenium arsenic photosensitive member, though having a long life compared with the organic photosensitive member, is, disadvantageously, a toxic substance and thus is difficult to handle. On the other hand, the a-Si photosensitive member, though being costly compared with the organic photosensitive member, is a harmless substance and thus is easy to handle. In addition, the a-Si photosensitive member has a high hardness and thus has excellent durability (which is five or more times greater than that of the organic photosensitive member), and characteristics thereof as a photosensitive member are hardly degraded even after long-term use, so that a high image quality can be maintained. The a-Si photosensitive member thus makes an excellent image bearing member whose running cost is low and that achieves a high level of environmental safety.
As is known, in an image forming apparatus using a photosensitive drum of any of the above-described types, due to characteristics thereof, depending on conditions of use, so-called image deletion is likely to occur, i.e. a faded image or an image smeared at a periphery thereof is likely to be formed. A factor responsible for the occurrence of image deletion is as follows. That is, when a surface of the photosensitive drum is charged by using a charging device, ozone is generated due to electrical discharge by the charging device. By the ozone thus generated, components contained in the air are decomposed to generate ion products such as NOx and SOx. Being soluble in water, these ion products adhere to the photosensitive drum and penetrate into an about 0.1 μm-thick roughness structure of the surface of the photosensitive drum. This makes it impossible for the ion products to be removed by using a cleaning system used in a general-purpose apparatus, and they take in moisture in the atmospheric air, which leads to a decrease in resistance of the surface of the photosensitive drum. Because of this, a lateral flow of potential occurs at an edge portion of an electrostatic latent image formed on the surface of the photosensitive drum, which may result in the occurrence of image deletion. This phenomenon is pronounced particularly in a case of the a-Si photosensitive member, which hardly suffers from surface wear caused by a blade or the like and whose surface has a molecular structure likely to absorb moisture.
Various methods for preventing the occurrence of such image deletion have conventionally been proposed. For example, a method is known in which a heat generating member (heater) is provided inside a photosensitive drum or inside a rubbing member being in contact with the photosensitive drum, and controlled, based on a temperature and a humidity detected by a temperature and humidity sensor in an apparatus, to perform heating to evaporate moisture adhering to a surface of the photosensitive drum, so that the occurrence of image deletion is prevented.
The method in which the heater is disposed inside the photosensitive drum, however, requires that a slider electrode be used to connect the heater to a power source. Due to the presence of this sliding portion that connects the heater to the power source, as a total length of time of rotation of the photosensitive drum increases, a contact fault occurs at the sliding portion, which has been disadvantageous. Furthermore, in these days when there is a growing need for measures directed toward energy saving and environmental protection, it is strongly demanded that power consumption at the time of standby and at the time of normal printing be reduced. Particularly an image forming apparatus of a type having a plurality of drum units, such as a tandem-type full-color image forming apparatus, is large in power consumption, and hence it is not desirable to incorporate a heater therein. Other methods include a method in which heat around a cassette heater or a fixing device is transmitted to a vicinity of a photosensitive drum. This method, however, is not efficient in that a developing device and so on in the vicinity also are undesirably heated.
As a solution to the above, an image forming apparatus is known that sets a weak charging period in which a charging voltage formed only of a direct current voltage or a charging voltage obtained by superimposing an alternating current voltage lower than that used at the time of image formation on a direct current voltage is applied, to a prescribed period before a start or after completion of a regular charging period or between a plurality of regular charging periods, thereby suppressing the generation of by-products of electrical discharge caused by application of a charging bias at a time other than the time of image formation.
Furthermore, an image forming apparatus is known that is capable of executing a moisture removing mode of performing, in order, a first moisture removing step in which, by using a cleaning blade, moisture is removed from a surface of a photosensitive drum, a second moisture removing step in which toner on a developing roller is conveyed toward the photosensitive drum and used to absorb moisture on the surface of the photosensitive drum, and the moisture is removed together with the toner, and a third moisture removing step in which moisture on a charging roller and on the surface of the photosensitive drum is removed by application of a voltage to the charging roller.